1. Field of the Invention
The present invention relates to a tracking servo system for une in an information recording disc playing system.
2. Description of Background Information
A tracking servo system is indispensable in a system for playing an information recording disc such as a video disc or a digital audio disc (hereinafter, simply referred to as a disc) in order to control the position of the information reading light spot so that it always accurately traces a recording track formed on the disc even if the disc is slightly eccentric.
The tracking servo system forms the so called closed loop control system in which a tracking error signal corresponding to an amount of deviation of the information reading light spot with respect to the recording track of the disc is generated and an actuator for deflecting the information reading spot in a radial direction of the disc is driven in accordance with the tracking error signal, thereby to control the position of the information reading light spot with respect to the recording track.
Furthermore, in such a servo system, during a track jump operation in which the information reading point is moved across the tracks the control operation is performed such that the servo loop is opened and jump pulses are applied to the tracking actuator, and subsequently the servo loop is closed at a suitable timing, so that the servo loop is locked in quickly.
In such a tracking servo system, an equalizing circuit capable of a proportional integral differential (PID) operation is incorporated in the servo loop, and the stabilization of the servo loop is attained by the operation of the equalizing circuit. An example of the gain and phase characteristics of the equalizing circuit is illustrated in FIG. 4 as servo open loop characteristics. As shown, the compensation operation of the equalizer is such that the gain at the low frequency range is boosted by an integration coefficient so as to improve the traceability of the pickup against the eccentricity of the disc. Furthermore, the phase advance is compensated for by means of the differential coefficient. However, the compensation operation is attained primarily by the integration component.
With this type of control system in which the integration component is principal, if it is assumed that the light spot S1 for reading information is moved while the tracking servo loop is closed, the tracking error signal after passing through the equalizing circuit effects an acceleration operation to the actuator during a first half period of a track tack jump operation shown as a period A in FIG. 2B, and a deceleration operation to the actuator during a last half period of the track jump operation shown as a period B in FIG. 2B. This results in that the lock in of the servo loop is performed during the acceleration drive of the actuator. However, this will cause a large overshoot, and the lock in of the servo system will become unstable.